/Engine/Source/Editor/UnrealEd/Classes/Editor/EditorEngine.h
Assigns a new label to an actor. If the name exists it will be appended with a number to make it unique. Actor labels are only available in development builds.
static void SetActorLabelUnique ( [AActor](API\Runtime\Engine\GameFramework\AActor) * Actor, const [FString](API\Runtime\Core\Containers\FString) & NewActorLabel, const [FCachedActorLabels](API\Editor\UnrealEd\Editor\FCachedActorLabels) * InExistingActorLabels )